fredman
Masterpiece
What exactly do you mean by 50-70% organic soil? Reason i'm asking, is my robur has been in inorganic soil since I had it. While it grew well, it suffered terribly with powdery mildew every year. Last year I did an experiment and tried a full organic soil on most my trees...sifted compost/worm castings and pine bark. It didn't really work as it dried very fast, and didn't wet easy when dry....I used to much organics. One thing I noticed though, was the robur had almost none of the mildew. Don't know if it's a coincidence, but i'm planning to give it some organic soil again this year to see if it happens again...was thinking to add 30-40% sifted compost to the bonsai mix...?I would not worry too much. Quercus robur grows well in free draining sand, but they don't mind swamps either.
A good coarse drainage layer is sufficient for my robur oaks, which are in a 50-70% organic soil for 2 years.